1. Field of the Invention
This invention relates to a downhole tool which can act as a variable stabiliser or a control for directional drilling.
2. The Prior Art
It is well known to provide apparatus for deflecting a portion of a drill string to impart a curve to the drill string in order to control the direction of drilling, or to control the deviation of the borehole from the initial centreline of the bore. It may be that it is desired to restrain deviation of the borehole from the initial centreline, or to increase the directional deviation. However, such known devices are often unreliable and uneconomic.
U.K. patent publications 2172324, 2172325 and 2177738 disclose a stabiliser comprising: a housing which is adapted to engage with a well bore by means of a wall contact assembly, such that the housing is coaxial with the well bore, a mandrel rotatable within the housing: and hydraulic actuator means for positioning the centreline of the mandrel relative to the longitudinal axis of the housing and of the well bore.
Prior art stabilisers have controllable positioning devices which are movable between a position in which the stabiliser is centred in the borehole and a position in which the stabiliser is offset from the centreline of the borehole. Each positioning device is movable between a first retracted position and a second extended position, but cannot be held at any intermediate position between the fully retracted and fully extended positions. Thus, such known stabilisers only provide relatively crude control for directional drilling and other related activities.